Ternary operation

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In mathematics, a ternary operation is an n-ary operation with n = 3. A ternary operation on a set A takes any given three elements of A and combines them to form a single element of A. An example of a ternary operation is the product in a heap.

In computer science a ternary operator (sometimes called a tertiary operator)^[1] is an operator that takes three arguments. The arguments and result can be of different types.

Many programming languages that use C-like syntax feature a ternary operator, ?:, which defines a conditional expression. Since this operator is often the only existing ternary operator in the language, it is sometimes simply referred to as "the ternary operator".

The ternary conditional operator was anticipated by ALGOL, which allowed if then else to be used in expressions, for example:

a := if x > 0 then x else -x.

Languages that emphasize functional programming, such as Scheme, may define the plain if-then-else construction in terms of this ternary operator. For example, the Scheme expression (if (> a b) a b) is equivalent in semantics to the C expression (a > b) ? a : b.

Though it had been delayed for several years by disagreements over syntax, a ternary operator for Python was approved as Python Enhancement Proposal 308 and was added to the 2.5 release in September 2006. Python's ternary operator differs from the common ?: operator in the order of its operands; the general form is op1 if condition else op2. This form invites considering op1 as the normal value and op2 as an exceptional case. Before 2.5, one could use the syntax (lambda x:op2,lambda x:op1)[condition]() which also takes care of only evaluating expressions which are actually needed in order to prevent side effects.

Version 9 of Visual Basic has added a ternary operator, If(), in addition to the existing IIf() function that had existed previously. As a true operator, it does not have the side effects and potential inefficiencies of the IIf() function.

The syntaxes of the tokens are similar - If(condition[, op1], op2) vs IIf(condition, op1, op2). As mentioned above, the function call has significant disadvantages, because the subexpressions must all be evaluated, according to Visual Basic's evaluation strategy for function calls and the result will always be of type variant (VB) or object (VB.NET). The If()operator however does not suffer from these problems as it supports conditional evaluation and determines the type of the expression based on the types of its operands.

[edit] Examples

[edit] In PHP

A traditional if-else construct would be written like so:

 if ($cake == "fresh") 
 {
     print "Yum yum! This cake is tasty.";
 } else 
 {
     print "Yuck! This cake tastes awful!";
 }

This can be rewritten as the following ternary statement:

 $message = ($cake == "fresh") ? "Yum yum! This cake is tasty." : "Yuck! This cake tastes awful!";
 print $message;

The statement takes the form of:

 $variable = condition ? if true : if false

[edit] In JavaScript

An if-else construct would be written like so:

var num = 5;
 
if(num == 5)
{
 document.write("Number was 5");
} else 
{
 document.write("Number was " + num);
}

This can be rewritten as a ternary statement:

var num = 5;
num == 5 ? document.write("Number was 5") : document.write("Number was " + num);

[edit] See also

• ?:, the ternary conditional expression
• IIf, inline if function

[edit] External links

• Description of Ternary Operator in C#
• Description of If operator in Visual Basic
• Description of Ternary Conditional Expression in Python (PEP 308)
• Alternative workaround for ternary conditional expression
• Description in the Java Language Specification
• Description in the PHP Language Documentation
• Visual Basic 9.0 Language Specification